As known, a gas discharge lamp has many benefits such as high brightness, long life, small volume, high lighting efficiency and good color rendering efficiency. Consequently, the gas discharge lamp is widely used in a variety of outdoor, indoor or automotive lighting devices. The gas discharge lamp is usually equipped with an electronic ballast for controlling the AC current that is outputted from the gas discharge lamp.
The conventional electronic ballast at least comprises a converter and an inverter circuit. The converter is controlled by a constant power control circuit. Consequently, the DC voltage received by the converter is converted into regulated DC voltages with different voltage levels. The constant power control circuit is also used for detecting a DC voltage and a DC current from the converter. According to the detecting result, the converter is controlled by the constant power control circuit to output a constant power. The inverter circuit is for example a full-bridge inverter circuit composed of four switch elements. Two switch elements at the upper bridge arm and two switch elements at the lower bridge arm are connected with each other in parallel. Under control of an inverter control circuit, the two switch elements at the upper bridge arm and the two switch elements at the lower bridge arm are alternately turned on or turned off. Consequently, the DC voltage and the DC current from the converter are converted into an AC voltage and an AC current, respectively.
As known, the simultaneous conduction of the two switch elements at the upper bridge arm or the simultaneous conduction of the two switch elements at the lower bridge arm may cause damage of the switch elements. For avoiding simultaneous conduction, after the on-state switch elements at the upper bridge arm and the lower bridge arm are switched to the off state for a certain time interval, the off-state switch elements at the upper bridge arm and the lower bridge arm will be switched to the on state. The certain time interval is also referred as a dead time. During the dead time, the two switch elements at the upper bridge arm and the two switch elements at the lower bridge arm are simultaneously in the off state. Moreover, an ignitor is connected between the inverter circuit and the gas discharge lamp for temporarily and largely increasing the voltage level of the AC output voltage from the inverter circuit, thereby driving illumination of the gas discharge lamp.
Since the operation of the gas discharge lamp is driven by the AC current from the electronic ballast, the quality of a current crest factor (CCF) of the AC current may directly influence the use life of the gas discharge lamp. FIG. 1 is a schematic timing waveform diagram illustrating the current of a gas discharge lamp driven by a conventional electronic ballast. During the polarity inversion of the AC current from the conventional electronic ballast, the inverter circuit is continuously operated to provide rated electric energy to the inverter circuit. Since the two switch elements at the upper bridge arm of the inverter circuit and the two switch elements at the lower bridge arm of the inverter circuit are simultaneously in the off state during the dead time, the output voltage from the inverter circuit is not generated during the dead time. Meanwhile, the electric energy outputted from the inverter circuit can be only stored in an output capacitor of the inverter circuit. After the transient polarity inversion of the output current from the electronic ballast, the predetermined electric energy from the inverter circuit and the electric energy stored in the output capacitor are simultaneously transmitted to the gas discharge lamp. Due to the transient high electric energy, the lamp current flowing through the gas discharge lamp may result in a peak current (see FIG. 1). The peak current also results in a peak voltage of the gas discharge lamp. Under this circumstance, the current crest factor is reduced, and thus the use life of the gas discharge lamp is shortened.
For solving the above drawbacks, the conventional electronic ballast may further comprise a detecting circuit for detecting whether the output current from the converter fluctuates. If the output current from the converter fluctuates, the detecting circuit issues a corresponding signal to reduce the output power of the converter in order to restrain the peak current. In other words, the conventional method of restraining the peak current is passively performed after the AC output current from the electronic ballast results in the peak current. Since the action of restraining the peak current is triggered when the peak current is generated, the peak current fails to be completely restrained and the efficacy of restraining the peak current is unsatisfactory. Moreover, since the detecting circuit needs to detect and judge current fluctuation, the computation is complicated and the circuitry configuration is costly.
Therefore, there is a need of providing an electronic ballast with a real-time current crest factor improvement function in order to eliminate the above drawbacks.